Archean

The eon following the Hadean, spanning from about 4.0 to 2.5 billion years ago, marked by the appearance of the first preserved continental crust, the earliest known life, and an atmosphere carrying little to no free oxygen. Includes the Archean eon itself and its four constituent eras in the standard geologic time scale, ordered oldest to youngest: Eoarchean, Paleoarchean, Mesoarchean and Neoarchean. Belongs here: the Archean eon and any era formally nested inside it. Does not belong here: the Proterozoic eon that follows, defined by the rise of atmospheric oxygen the Archean itself predates, or the Hadean eon that precedes it, with no preserved rock record of its own.